Expertise

Biography

Fred Tillman joined the USGS Arizona Water Science Center in Tucson, AZ in 2006 after two-plus years as a National Research Council (NRC) fellow with the USEPA Office of Research and Development (ORD) in Athens, Georgia. His current projects include monitoring the extent of 1,4-dioxane and chromium contamination in groundwater at the Tucson Airport Remediation Project (TARP) Superfund Site, investigating potential changes in groundwater recharge under projected climate change in the upper Colorado River basin, and understanding the potential for uranium mining impacts on regional water resources in the Grand Canyon area.

Prior to joining the USGS, Fred was a post-doctoral researcher with the Ecosystems Research Division of USEPA’s ORD from 2004 through mid-2006, working with Dr. James W. Weaver on issues related to soil vapor intrusion, particularly from leaking underground storage tanks. Results from this research are documented in several journal and web publications (see below). Fred joined EPA after completion of his doctoral degree in Environmental Engineering at the University of Virginia in 2003, where he worked with Dr. James A. Smith on the response of volatile organic compound (VOC) vapors in the unsaturated zone to natural barometric pressure fluctuations (barometric pumping).

Tillman, F.D, and Anning, D.W., 2014, A data reconnaissance on the effect of suspended-sediment concentrations on dissolved-solids concentrations in rivers and tributaries in the Upper Colorado River Basin. Journal of Hydrology, 519, p. 1020–1030. [Link]

Tillman, F.D, Choi, J.-W., and Smith, J.A., 2003, A comparison of direct measurement and model simulation of total flux of volatile organic compounds from the subsurface to the atmosphere under natural field conditions. Water Resources Research, 39, no. 10, p. 1284–1294. [Link]

Tillman, F.D, 2003, Design and testing of a chamber device to measure total flux of volatile organic compounds from the unsaturated zone under natural conditions. Charlottesville, University of Virginia, Ph.D. dissertation, 130 p.

Science and Products

Studies of Sources and Transport of Dissolved Solids (Salt) in the Colorado River Basin using the Spatially Referenced Regressions on Watershed Attributes (SPARROW) Model

The Upper Colorado River Basin (UCRB) encompasses about 112,000 mi2 and discharges more than 6 million tons of dissolved solids (salt) annually to the lower Colorado River Basin. About 45 percent of this...

The Colorado River has been identified as the most overallocated river in the world. Considering predicted future imbalances between water supply and demand and the growing recognition that base flow (a proxy for groundwater discharge to streams) is critical for sustaining flow in streams and rivers, there is a need to develop methods to better quantify present-day base flow across large...

Beginning in the late 1970s, 10- to 15-year cyclical oscillations in salinity were observed at lower Colorado River monitoring sites, moving upstream from the international border with Mexico, above Imperial Dam, below Hoover Dam, and at Lees Ferry. The cause of these cyclical trends in salinity was unknown. These salinity cycles...

Hydrologic model input datasets such as climate, land use, elevation, soil, and geology information are available in a range of scales for use in water resources investigations. Smaller spatial and temporal scale input data allow groundwater recharge models to simulate more physically realistic processes and presumably result in more accurate...

Elevated concentrations of dissolved-solids (salinity) including calcium, sodium, sulfate, and chloride, among others, in the Colorado River cause substantial problems for its water users. Previous efforts to reduce dissolved solids in upper Colorado River basin (UCRB) streams often focused on reducing suspended-sediment transport to streams, but...

Industrial activities causing extensive groundwater contamination led to the listing of the Tucson International Airport Area (TIAA) as a Superfund Site in 1983. Early groundwater investigations identified volatile organic compounds (VOCs), including the chlorinated solvents trichloroethylene (TCE) and perchloroethylene (PCE), in wells in the area...

The Colorado River is an important source of water in the western United States, supplying the needs of more than 38 million people in the United States and Mexico. Groundwater discharge to streams has been shown to be a critical component of streamﬂow in the Upper Colorado River Basin (UCRB), particularly during low-ﬂow periods. Understanding...

The processes that affect water chemistry as the water flows from recharge areas through breccia-pipe uranium deposits in the Grand Canyon region of the southwestern United States are not well understood. Pigeon Spring had elevated uranium in 1982 (44 μg/L), compared to other perched springs (2.7–18 μg/L), prior to mining operations at the nearby...

An investigation of the change in groundwater recharge in response to potential climate change
was performed for the UCRB using the SWB groundwater recharge model and downscaled
climate data from the CMIP5 multi-model dataset. Climate projections from 97 downscaled
CMIP5 datasets were assumed to be equally likely and recharge simulation results...

Understanding groundwater-budget components, particularly groundwater recharge, is important to sustainably manage both groundwater and surface water supplies in the Colorado River basin now and in the future. This study quantifies projected changes in upper Colorado River basin (UCRB) groundwater recharge from recent historical (1950–2015)...

Potential health effects from hexavalent chromium in groundwater have recently become a concern to regulators at the Tucson International Airport Area Superfund site. In 2016, the U.S. Geological Survey sampled 46 wells in the area to characterize the nature and extent of chromium in groundwater, to understand what proportion of total chromium is...

Population growth in the Verde Valley in Arizona has led to efforts to better understand water availability in the watershed. Evapotranspiration (ET) is a substantial component of the water budget and a critical factor in estimating groundwater recharge in the area. In this study, four estimates of ET are compared and discussed with applications...

The Colorado River and its tributaries supply water to more than 35 million people in the United States and 3 million people in Mexico, irrigating more than 4.5 million acres of farmland, and generating about 12 billion kilowatt hours of hydroelectric power annually. The Upper Colorado River Basin, encompassing more than 110,000 square miles (mi2...

Simulating groundwater flow in basin-fill aquifers of the semiarid southwestern United States commonly requires decisions about how to distribute aquifer recharge. Precipitation can recharge basin-fill aquifers by direct infiltration and transport through faults and fractures in the high-elevation areas, by flowing overland through high-elevation...